Undergoing joint replacement surgery can make the difference between living in pain or doing the activities you enjoy. The goal of the surgery is to reduce or eliminate pain in the diseased joint and give you a new joint that allows you essentially pain free movement for many years.

    Due to new advancements in joint replacement technology, many people are opting for this surgery at a younger age. With this in mind and for the benefit of anyone who has a joint replacement, it is important that we continually evaluate the success and risks associated with joint replacement surgery, short and long term.

    For years, improvements have focused on specific areas of the surgery. Experts have evaluated fixation of the artificial joint’s components in the host bone, the refinement of cement and cementing techniques, biological fixation of titanium and similar materials, the effect of beads or surface roughness on the implant interface with bone, ingrowth techniques and the design of many artificial joints. Having achieved a high degree of success through new innovations and upgrades, the current focus centers on the design of the implant and how it wears long term.

    Historically, loosening of the replaced joint from the host bone had been the primary reason for revision surgery. With total joint replacements lasting longer, the issue of wear is now the number one reason joints may require re-operation. Our goal now is to reduce the degree of wear between the moving surfaces of the artificial joint components so they last even longer.

   Over many years, different materials including precious metals, glass, ivory, Teflon and other synthetic materials have been tried and/or evaluated. Now, three materials are routinely used for bearing surfaces: 1) high-density polyethylene (with and without radiation treatment), 2) metal, commonly a stainless steel alloy which is harder than stainless steel, and 3) ceramics.

    Because the need for hip and knee replacement surgery is greater than shoulder replacement, research has more focused on hip and knee joints. Due to the differences of stresses and movements of different joints, artificial joint materials wear differently. Because of these significant differences, I feel each joint should be addressed separately.

The Hip

   Total joint replacement of the hip has been performed at least ten years longer than that of the knee. Since the hip is a ball-and-socket joint, the greatest stresses result from rotation, these stresses are directly related to weight and muscle strength. Early hip replacements performed usually included a small metal ball being placed into a thick polyethylene (plastic) socket. Many of these surgeries have lasted more than 20 years. We now know that the wear of polyethylene in the joint frequently results in a significant amount of debris formation. Over time, this reaction can loosen the bond between the metal components and the bone around them. This long-term process of loosening can sometimes cause local bone loss necessitating a revision to exchange the components. Bone grafting is sometimes necessary depending on the amount of bone loss. Occasionally, these changes are severe in nature, requiring extensive bone grafting with major revisional surgery. This is the primary reason why routine follow-up, including undergoing new x-rays, is recommended annually once you’ve undergone joint replacement surgery.

    The materials used in joint replacement surgery and the relationship between these materials including the wear factors are continuously scrutinized. As a result, we’ve updated the ball size from a 22 mm size up to a 44 mm ball diameter because it reduces the risk of dislocation. We’ve upgraded the plastic component to a higher-density with radiation to make the plastic polymers link together more strongly; thus, decreasing wear over time. To minimize wear and debris formation, metal on metal and ceramic implants have been developed as well as the ball being altered in size. The development of new materials has significantly reduced wear and debris formation in the hip joint. I feel these material changes will further improve the longevity of the joint implants used in hip replacement.

The Knee

The knee has a different wear pattern. In addition to weight and rotation, the knee joint glides as a hinge.
Because of the differences between the hip and knee, surface materials cannot always be exchanged with the materials used in the hip. Some of the polyethylene changes are being used in the artificial knee joints, but more changes are still expected. Like the hip, ceramic material has been developed as an alternative to the metal component in the knee but the fixation method requires cementing. More changes are likely in this particular design. Metal-on-metal alternatives are not yet available in knee replacement. Continued analysis of the knee component designs has already extended the lifespan of the knee replacement. We now know that moveable parts of the artificial knee does reduce the individual wear in each of the components used, thereby enhancing the lifespan of these components. These components appear to be very successful. Continuing studies, which examine outcomes of current surgery, helps us predict the results of the future.

The Shoulder

    Because the shoulder joint is not a weight-bearing joint, it is not subjected to as much weight and is more of a ball on a socket versus a hall in a socket. The shoulder provides much more sliding motion than the hip.
Due to the shoulder’s “non-weight bearing” status, the shoulder components are more affected by activity than by weight factors. The Gold standard of the shoulder replacement continues to be the use of the metal-on-plastic principle. I feel confident that the improved quality of the polyethylene will help increase this joint’s longevity. This improvement as well as new designs will help decrease bone loss at the time of surgery, increase shoulder stability and extend wear of the components.

In Summary

   Early results in joint replacement surgery have helped direct efforts of total joint replacement surgery improvements. Today, joint replacement has been consistently successful 90-95% of the time and is now lasting between 12—20 and more years. The “weak link” continues to be the wear factor. With continued improvement of the materials we aim at doubling the expected lifespan of these new joints. Major factors of wear continue to be body weight and activity. These concerns must be addressed prior to surgery with understanding and education for the best chance at long term success. Gliding activities such as walking, bicycling, golfing and bowling are better in the long run for the patient and his/her joint replacement then high-impact loading exampled by running, jogging and jumping.

New Horizons

   New approaches and procedures continue to come our way. In some cases, to minimize soft-tissue trauma during surgery, we are turning to minimally invasive surgery resulting in a smaller incision. Future enhancements include the use of three-dimensional enhancement whereby the surgeon utilizes computerized guidance during surgery. We continue to research and develop ways of further preventing blood clot formation, blood loss and other complications. In all probability, the future of joint replacement surgery will include computer-guided surgery with synthetic materials. The benefits will be minimal blood loss and complications.

   Most of our discussion has been focused on osteoarthritis in major joints. Other significant factors involved in the long-term success of total joint replacement including inflammatory arthritis (i.e. rheumatoid, psoriatic), metabolic diseases (gout, pseudogout) have not been addressed in this article. They should be considered on an individual basis. Once your joint has been replaced, individual care is a very important factor in keeping your joint in good condition and furthers the research, which evaluates the long-term success of the surgery.

About Dennis Armstrong, M.D.
Dennis Armstrong, M.D. graduated from Wayne State School of Medicine and completed his orthopaedic residency at Henry Ford Hospital in Detroit, Michigan. Dr. Armstrong is board certified in orthopaedic surgery and specializes in arthritis reconstructive surgery. Dr. Armstrong has been involved in several clinical investigative studies researching and caring of those undergoing total joint replacement. He has presented numerous papers and provided multiple presentations on his clinical research. Dr. Armstrong has been in practice for more than 20 years in the East Valley of the Phoenix Metro area in Arizona.

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